Is it possible to get melanoma skin cancer in your armpit where the sun never shines?

Yes: Melanoma can occur in non-sun exposed skin.

Yes.: Although risk factors include sun exposure and fair complexion, patients with dysplastic nevus syndrome (an autosomal dominant condition with many nevi), and certain kinds of melanoma , like acral lentiginous can possibly get malignant melanoma even without sun exposure. Anoyone with a nevus, or nevi should be aware, especially if it is irregular shaped, multicolored and asymmetric.

Unfortunately yes: Melanomas are most commonly caused by sun exposure, but they rarely do appear in strange places...I have seen/known patients who have had melanomas come up under their fingernails, in the mouth or sinuses, etc. If you notice any moles on your body which are changing, have them looked at.